Oil shale is generally comprised of an inorganic matrix of rock minerals and organic bitumen and kerogen trapped within the inorganic matrix. There is no oil or petroleum per se in "oil shale" which is a misnomer. Petroleum is contained in certain oil-bearing shales and may be separated therefrom by mechanical means, but such oil-bearing shales are to be distinguished from "oil-shale" as the term is used herein.
The composition of the inorganic matrix of oil shale varies widely but is generally comprised of major amounts of carbonates, such as dolomite and calcite, and silicates, such as quartz and feldspar, and minor amounts of clays, sulfurbearing pyrites and miscellaneous inorganic minerals.
The organic bitumen and kerogen in oil shale have value as a fuel and may be converted into other fuels. This organic matter is present from trace amounts to over 60% by weight with kerogen forming the major portion thereof. The major portion of these organic materials do not exist in any significant quantities outside the inorganic matrix which thereby prevents oil shale from being utilized as a fuel unless it is further processed to separate the organic materials, referred to hereinafter as "organics," from the inorganic matrix.
The most common commercial method for recovering organics from oil shale is by retorting, also referred to as pyrolysis or destructive distillation, wherein the oil shale is crushed and heated to high temperatures to break the bonds between the kerogen and the inorganic matrix and to crack the complex kerogen molecule into simpler components. By this technique about 66% of the organics are converted into a crude shale oil, about 9% are converted into a petroleum gas, and the remaining 25% is unrecovered and remains as a carbon enriched residue. A portion of the recovered organics may also be used as a source of fuel to support the pyrolysis, but if it is, it represents a loss of potentially recoverable organics.
Pyrolysis of oil shale creates an enormous problem in material handling and disposal of spent shale. For example, for every ton of shale containing 25 gallons of kerogen per ton, pyrolysis will produce approximately 200 pounds of oil, 27 pounds of retort gases, 75 pounds of carbonaceous residue, and 1700 pounds of denuded, spent shale rock. A plant designed to produce 50,000 barrels of kerogen per day would require 84,000 tons of rock shale per day and would produce approximately 70,000 tons of spent shale which would render impractical any large scale commercial operation.
Another problem with pyrolysis is that the large quantities of sulfur and nitrogen present in the organics will be converted and passed into the atmosphere as sulfur dioxide and the oxides of nitrogen which are objectionable air pollutants.
Another approach for recovering organics from oil shale is to dissolve the inorganic matrix of the oil shale in an aqueous acid solution to release the organics and recover them by flotation. This method, however, has generally been limited to laboratory research use and has not been developed as a practical commercial means for recovering organics in situ because of the danger to life and to the environment involved in handling acid, and because of the economics involved in transporting and handling acid for use in situ.
A solvent removal system is another possible approach for recovering organics from oil shale, but this is not feasible inasmuch as kerogen, which is the major component of such organics, is insoluble in organic and aqueous solvents.
It is, therefore, an object of the present invention to convert oil shale into a form in which the organic content thereof may be useable as a fuel, or may be converted into other forms of fuels such as petroleum-type oil or a synthetic natural gas, without the problems and disadvantages associated with the aforesaid methods. Other objects and advantages of the invention will become apparent as the specification proceeds.